1. Field of the Invention
The present invention relates to an image reading apparatus for supplying image data to a digital image processing system, such as a digital copying machine, facsimile, and printer, and more particularly to the adjustment of black levels in the output signals of the line sensors in an image reading apparatus having a plurality of line sensors.
2. Prior Art Description
A so-called close contact line sensor has been known as a device to optoelectronically read a distribution of densities on an image of an original. In such a line sensor, an array of a number of photo diodes picks up the density information on the image and converts the density information into electrical quantities that are expressed in the form of the amount of signal charges. The signal charges are temporarily stored, and transferred to a CCD shift register. Through the shift register, the signal charges are successively outputted in the form of video signals. In this case, it is unpractical to continuously form the line sensor as a single linear chip on the same substrate with a uniform sensitivity over its full length, when taking into account wafer size, production yield and cost to manufacture. A practical measure has been taken for this. In the measure, a plurality of chips are arrayed in line in the main scan direction, the image information along one line are dividedly read by the line sensors, and are transferred through channels. The read data are processed for each channel.
The photoelectric conversion characteristic of the line sensor differs for each picture element (pixel) and for each chip. Accordingly, a density obtained when one line sensor reads an image of an original is different from that when another line sensor reads the same image. The fact will be described using a case that the amount of light applied to the line sensor is zero, viz., the line sensor produces a dark output. As best illustrated in FIG. 48(a), the dark outputs outputted through five channels of a line sensor array consisting of five sensor chips are discontinuous in signal level among the sensor chips. If the original image is read by such a sensor array, viz. involving nonuniform signal levels, the resultant image suffers from stripes or density irregularity.
To cope with the problem, a measure conventionally taken is to average the dark voltages of all of the chips for each chip, and to adjust related circuit parameters so that each average value is equal to a target value D.sub.R shown in FIG. 48(b).
Actually, the data voltage outputted from the line sensor array in each channel gently increases as shown in FIG. 48(c). One of the causes for the incremental variation of the dark voltage is a nonuniformity of temperature in the chip. It has been known that the dark voltage is generally doubled every 8.degree. C. rise of temperature. If the above adjustment is applied to the sensor array exhibiting such a characteristic, a level difference .DELTA.D appears at each channel boundary as shown in FIG. 48(d). The level difference appears in the form of stripes or density irregularity on the reproduced image.